Fly ash discharged from a municipal refuse incineration plant is regulated, by 1992 legislation, to be treated to prevent leaching of heavy metals before final disposal. We have developed a stable and simple fly ash treatment method “Acid Extraction-Sulfide Stabilization Process (AES Process)”, in which the fly ash is acid-extracted and stabilized by NaHS to change the residual heavy metal into insoluble heavy metal sulfide. The results of many fundamental experiments and pilot tests indicate that it is important to consider the characteristics of fly ash (fine, scattering, moisture-absorbing, etc.) in designing the AES Process.